1. Field of the Invention
The present invention relates to a 3GPP2 system, and more particularly, to a method for configuring a transmission chain in a 3GPP2 system for supporting a flexible or variable data rate of an information bitstream.
2. Background of the Related Art
In general, the 3GPP2 (the 3rd Generation Partnership Part 2) system has two transmission modes, specifically a flexible data rate mode and a variable data rate mode, besides a regular data rate mode. The regular data rate mode is a transmission mode operative on a fixed chain called a Radio Configuration (RC). The RC represents a kind of transmission chain in which lengths of information data, channel interleaver, and an output bitstream from a channel encoder according to a code rate of channel codes are made the same to form a standard. In this instance, there is a certain standard rule among the size of the channel interleaver, the coding rate of the channel encoder, and the length of a channel Walsh code. That is, once a chip rate to be used is fixed, a number of chips required for one modulated symbol are fixed according to the size of the channel interleaver, which may be defined as a spreading factor. The length of the Walsh code that can subject different channels to code multiplexing is fixed according to the spreading factor. A number of available Walsh codes are proportional to the length of the Walsh code. Therefore, a number of channels a multiplexing channel can accommodate vary with the Walsh code.
Let us suppose lengths of input information bitstreams of the same lengths after being subjected to channel coding process. In this instance, a capability of error correction code for correcting possible errors in the channel becomes stronger, as the coding rate of the channel encoder is lower. That is, the lower the coding rate of the channel encoder, the stronger error correction capability. This permits use of the lower transmission power.
However, use of the channel encoder with a low code rate elongates the output bitstream of the channel encoder, which, in turn, elongates the size of the channel interleaver. This ultimately increases a modulation symbol rate, and reduces a number of chips required for one modulation symbol at a fixed chip rate, and reduce a number of useful Walsh codes. Opposite to this, if a high channel coding rate is applied to input bitstreams of the channel encoder of identical lengths, lengths of output bitstreams of the channel encoder become short even if the error correction capability is low. This decreases the modulation symbol rate, and permits use of a short channel interleaver, to increase a number of available Walsh codes.
From the above description, it is clear that there is a certain trade off between the coding rate of the channel encoder and the Walsh code spacing. The RC is a standardized transmission chain, which is useful when securing the Walsh code spacing is preferable or when a lower transmission power is required, takes such a trade off relation into account. In the 3GPP2 system, there are currently several standardized RCs for 1× system of 1.2888 Mcps chip rates and several standardized RCs for 3× system of 3.686 Mcps chip rates. It should be noted that since the spreading factor has a value in a form raised to the second power, both the input data rate and the size of the interleaver defined in each of the RCs also have forms raised by two times.
Before a channel is formed between a mobile station and a base station, the mobile station and the base station determine the RC to be used and the spreading factor on the RC, i.e., the size of the channel interleaver, through a negotiation for processing a communication matched to the chain. In modes having a transmission chain different from the transmission chain defined in the RC, the flexible data rate mode and the variable data rate mode are used. In the flexible data rate, any data transmission rate other than the standard data transmission rate supported on the RC can be supported. The flexible data rate has been introduced to support an Adaptive Multi-Rate (AMR) codec, one of 3GPP2 speech codec in a 3GPP physical layer. That is, in the case of AMR, data bits not consistent to the standard transmission rate supported on each of the 3GPP2 RC can currently be produced for a 20 ms frame period.
On the other hand, the variable data rate mode has been provided under the following purpose. In the 3GPP2 system, the base station makes transmission scheduling to a forward supplemental channel, when the base station assigns a fixed data transmission rate to the mobile station for a time period by means of a message. However, a channel situation between the base station and the mobile station during the particular time period may be changed, and a system load on the base station may be changed. For example, as the mobile station goes farther from the base station, the channel situation becomes poorer, causing the base station to lack enough transmission power to transmit data to a particular mobile station at the present data transmission rate.
To solve this problem, the base station may stop transmission of data to supplementary channels for the time period. However, such a solution causes delay in data transmission, as well as needless waste of available transmission power and Walsh codes. As an alternative, the base station may make re-scheduling of the data transmission rate after a time period has passed. However, this alternative also causes the problem of the time delay and the waste of the Walsh codes. This situation occurs not only in the forward link The channel situation between the mobile station and the base station may vary with movements of the mobile station in a reverse link, causing lack of transmission power required for sustaining an appropriate quality. Consequently, the variable data rate mode is used to solve such a situation. In the variable transmission rate, the transmission rate is varied with frames depending on situation. That is, if the channel environment is determined to be poor, the base station drops the transmission rate of the supplementary channel, and if the channel environment is determined restored, the base station restores the transmission rate of the supplementary channel. Provided that such a variable data rate mode is applied, the base station can use available power without the frequent re-scheduling.
To support the flexible data rate mode and the variable data rate mode, each of the RCs in the present 3GPP forms a transmission chain using the following methods.
As described before, the length of the channel interleaver used in each RC is fixed according to the spreading factor. Since the spreading factor has a value in a form raised by second power, a ratio of the size of the interleaver fixed according to one spreading factor to the size of the interleaver fixed according to another spreading factor one step lower than the one spreading factor is 1:2, exactly. If the greater spreading factor is represented with ‘A’ and the smaller spreading factor is represented with ‘B’, a 1:1 mapping can be established between the spreading factor and the input information bitstream of the channel encoder in each RC. If lengths of the input information bitstreams of the channel encoder are represented with IA for the spreading factor ‘A’ and IB for the spreading factor ‘B’ respectively, a relation of IB=2*IA is established. If the sizes of the channel interleavers to be used are represented with NA and NB respectively, a relation of NB=2*NA is established.
If it is assumed that a coding rate of the channel encoder (using turbo codes or convolution codes) in the RC is 1/n as illustrated in FIG. 1, taking the flexible or variable data rate mode into account, in which ‘I’ (a length of an information bitstream having CRC bits, tail bits, and reserved bits added thereto in steps 10 and 11) is a length of the input bitstream of the channel encoder, that meets a non-regular data length of “IA<I<IB”, the input ‘I’ will provide an output “n*I,” when “NA<n*I<NB” is also met (step 12). Consequently, a certain operation for matching the length of the output bitstream of the channel encoder “n*I” to the size of the interleaver is required. A method the 3GPP employs presently is that the length L (=n*I) of the output bitstream of the channel encoder is matched to the interleaver of N=NB to require a bit repetition as many as “NB−n*I” (step 13) according to the following uniform repetition process. That is, a (k)th output symbol of a repetitive block can be predicted starting from a code symbol of
      (          ⌊              kL        N            ⌋        )    ⁢  thinput bitstream for an index k increased from ‘0’ up to ‘N−1’.
Next, a method for supporting the variable data rate mode will be described.
In the variable data rate mode, a highest data rate supportable in the initial negotiation process, a data rate one step lower than the highest data rate, and data rate two steps lower than the highest data rate are defined as a transmission data rate set. Accordingly, in the variable data rate mode for the present supplementary channel, the data transmission rate can be adjusted within the supportable highest transmission rate set and the two step lower transmission rate. For the forward channel, the mobile station can only determine a rate variation by means of blind rate detection. Therefore, if a range of a data transmission rate variation is too large, an operative complexity of the mobile station is increased. The sizes of the channel interleaver and the Walsh codes, used in the highest transmission rate, should not be changed. That is, the interleaver and the Walsh codes used in the highest transmission rate presently should not be changed. If the data transmission rate is dropped to a half of the highest transmission rate, two times of symbol repetition is required for matching the size of the interleaver to the length of the output bitstream of the channel encoder. Similarly, if the data transmission rate is dropped to a quarter of the highest transmission rate, four times of symbol repetition is required for matching the size of the interleaver to be used on the channel and the length of the output bitstream of the channel encoder.
The foregoing example is applicable to a case of forward supplementary channel of a non-flexible data rate. The variable data rate mode for the flexible data rate can be supported in the supplementary channel. However, in this case, the definitions of the flexible data rate and the variable data rate mode themselves become vague. That is, even if the highest data rate in the variable data rate mode is a regular rate in the present RC and the one step lower data rate is also a data rate fixed in the present RC, the one step lower data rate can be taken as a flexible data rate that uses no transmission chain fixed in the present RC. This is because the size of interleaver in the variable data rate mode, i.e., the spreading factor, is fixed to that of the highest transmission rate.
As one example, take a RC4 of the present forward channel when the turbo codes or convolution codes of 1/2  rate is used. It is assumed that the highest transmission rate useable in the variable data rate mode is 76.8 kbps, when the size of the interleaver used in the forward RC4 is fixed at 3072. A variable data rate method in this mode will be discussed. It is assumed that an appropriate value from a set of usable data transmission rates {19.2 kbps, 38.4 kbps, and 76.8 kbps} is used. Though data transmission rates of 19.2 kbps, and 38.4 kbps are the transmission rates defined in the RC4 undoubtedly, a problem lies in that there is no chain that connecting the transmission rates to the 3072, the present interleaver length, in the RC. Therefore, those transmission rates may be taken as the variable data rates.
Naturally, if the chain has an interleaver length fixed at ‘N’ even in the variable data rate mode, and the present data transmission rate is not defined in the present RC, the length of output bitstream of the channel encoder and the fixed interleaver size can be matched by means of the aforementioned uniform repetitive algorithm. That is, the flexible data rate and the variable data rate should be treated in a similar manner, and a method for supporting them can be explained with reference to a chain illustrated in FIG. 1.
However, the related art method for supporting the flexible data rate mode and the variable data rate mode makes concepts of the flexible data rate mode and the variable data rate mode in the existing RC somewhat vague. As explained, the concept of RC may be taken as a kind of standardized rule that defines a relation among the information data rates. However, in the flexible data rate mode, or in the variable data rate mode, the relation can not be established as a standardized rule in the RC. According to this, the size of the interleaver to be used is fixed, channel encoding is carried out according to the coding rate used in the present RC, and the code symbol repetition process is carried out for matching the encoder output with the fixed interleaver length. Thus, in such a flexible data rate mode and variable data rate mode, there is no more standardized relation between the coding rate of the channel encoder and the spreading factor in the RC.
In the related method, as discussed before, the code symbol is repeated for matching the encoder output with a particular spreading factor. It should be noted that ‘I/N’ and 1/n have the following relation at all data rates except standardized chains in the RC in the flexible data rate mode or in the variable data rate mode, where ‘I/N’ denotes an effective code rate, ‘N’ denotes an interleaver length, ‘I’ denotes the input information bitstream length of the channel encoder, and ‘1/n’ denotes a code rate defined in each RC.
                              I          N                <                  1          n                                    (        1        )            
In other words, equation (1) implies that the effective code rate is dropped in the flexible data rate mode, or the variable data rate mode, and the symbol repetition is used for matching with the interleaver, both of which imply that, though the effective code rate is reduced, an actual code rate is the 1/n defined in the RC as before.
Therefore, both the present flexible data rate mode and the variable data rate mode have a problem of not taking advantage of the coding gain attainable by reducing the coding rate. That is, if the interleaver size is fixed to a certain value, and the transmission rate of the data to be transmitted presently is fixed, it is necessary to select a coding rate that can provide the greatest coding gain according to a relation of the two, and to form a new transmission chain for carrying out a rate matching puncturing or data matching repetition to match the encoder output and the channel interleaver size.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.